Physical vapor deposition methods are used extensively for forming thin films of material over a variety of substrates. One area of extreme importance for such deposition technology is semiconductor fabrication. A diagrammatic view of a portion of an exemplary physical vapor deposition apparatus 10 is shown in FIG. 1. Apparatus 10 comprises a target assembly 12 which includes a backing plate 14 having a sputtering target 16 attached thereto and a cooling plate 15 attached an opposing side of the backing plate. Alternatively, some prior art assembly configurations lack the cooling plate (not shown).
Typically, apparatus 10 will include a substrate holder 18 for supporting a substrate during a deposition event. A substrate 20 such as a semiconductive material wafer is provided to be spaced from target 16. A surface 17 of target 16 can be referred to as a sputtering surface. In operation, sputtered material 24 is displaced from surface 17 of the target and deposits as a thin film onto surfaces of the sputtering chamber including the substrate, resulting in formation of a thin film 22.
Sputtering within system 10 is most commonly achieved within a vacuum chamber by, for example, DC magnetron sputtering or radio frequency (RF) sputtering. During a sputtering event, particle impingement upon surface 17 not only ablates material from such surface but additionally results in target heating. Accordingly, target cooling becomes important in order to maintain the integrity of the target and target assembly, and to maintain production of uniform and high-quality thin films.
Various materials including, metals, alloys and ceramics are deposited utilizing physical vapor deposition. Common target materials include, for example, aluminum, titanium, copper, tantalum, nickel, molybdenum, gold, silver, platinum, and alloys thereof. Sputtering targets are typically made of a high purity material. Since many high-purity materials are low strength, backing plates can be attached to the targets to provide support, especially for applications where the target is under pressure exerted by a cooling system.
Conventional backing plates are typically formed from copper, copper alloys (e.g. CuCr, CuZn), or aluminum alloys (e.g. Al6061, Al2024). These materials are typically chosen due to their thermal, electrical and/or magnetic properties. Aluminum alloys can have up to three times lower density than copper alloys but also can have a weaker Young's modulus.
In order to provide target cooling, conventional systems typically employ water cooling where water is either provided in a reservoir located behind the backing plate, between backing plate 14 and cooling plate 15, or between the backing plate and the target. However, conventional cooling systems are often of limited effectiveness and can be problematic.
In assembly configurations where the backside of the backing plate is exposed to the water, cooling efficiency can be limited due to the distance between the water and the target. In alternative conventional target configurations, cooling utilizes water channels along a backside of the target (between the target and backing plate) or by providing channels within an insert (not shown) disposed between a target and the backing plate. An exemplary conventional backing plate 14 for a channel cooled target assembly is depicted in FIG. 2.
Conventional backing plate 14 is configured to have a plurality of narrow channels or openings 36 across a front side 32 of the backing plate, where front side refers to the side of the backing which will interface a target within a target assembly. Backing plate 14 has a peripheral region 30 and is shown to comprise a plurality of bolt holes 31 which can be utilized to attach the backing plate to the target. It is to be understood that the backing plate depicted in FIG. 2 is an exemplary configuration and that alternative methods of attaching the target to the backing plate such as soldering or diffusion bonding can be utilized. However, bonding and/or soldering techniques can be problematic due to differences in thermal expansion between target and backing plates, which can result in bond failure and/or water leakage from between the cooling plate and target.
The plurality of narrow parallel channels of the conventional backing plate shown in FIG. 2 can additionally be problematic due to high pressures for producing water flow through the channels. Referring to FIG. 3, such shows a cross-sectional view of backing plate 14 and illustrates a water inlet 38 through the backside 34 of backing plate 14 in the peripheral. One of the narrow channel 36 is depicted across front surface 32 through which water passes, exiting the backing plate through an outlet 40 through the backside 34. Due to the limited water flow and high pressure during sputtering events, the backing plate configuration depicted in FIGS. 2 and 3, can result in water leakage, target damage/or and target deformation.
As an alternative to the channeled backing plate configuration, some conventional target assemblies utilize three or more components including a target attached to an insert or membrane which is connected to a backing plate. In addition to having low capacity water flow, these multi-component designs can be complicated and can result in misalignment during sputtering resulting in water leakage. Additionally, conventional methods of joining the target and backing plate can contribute to target warping and leakage problems.
It is desirable to develop alternative target assembly configurations and alternative cooling methods for sputtering deposition.